1. Field of the Invention
The present invention is directed to tooling systems and methods whereby thin film magnetic recording head wafers are fabricated into optimum size work pieces to eliminate high impact process steps while minimizing wafer stress-related colinearity distortion of rows of magnetic transducers.
2. Description of the Related Art
Many direct access storage device manufacturers employ thin film magnetic recording heads. In manufacturing such heads, rows of magnetic recording transducers are deposited simultaneously on wafer substrates using semiconductor type process methods. Subsequent to these depositions, the wafers are fabricated into rows of single element heads called slider rows. When separated from the slider rows, each slider contains magnetic read/write components and an air-bearing surface configured to aerodynamically "fly" over the surface of a spinning magnetic disk medium. FIG. 1 illustrates two back-to-back slider rows and an individual slider formed therefrom in accordance with conventional techniques. The slider air-bearing surface "A" has a slight crown angle that imparts aerodynamic properties.
The slider rows are bonded to tool blocks called transfer tools. Transfer tools provide a mechanism for holding the row of sliders while lapping or grinding operations are performed to produce an air bearing surface. Typically the slider rows distort from a colinear line according to the internal stress of the wafer material and the surface stresses developed when reducing the wafers to slider rows. Further distortion of the rows of sliders from a colinear line can occur as a result of the tool bonding operation. The combined stress distortion and bonding distortion of slider rows results in a total distortion or curvature condition called row bow. This row bow condition can detrimentally affect critical head performance parameters. For those skilled in the art of thin film magnetic head fabrication, the critical performance parameters which can be affected are commonly known as stripe height in mageto-resistive heads and throat height in inductive heads. As implied by their nomenclature, it is the height of each that must be tightly controlled for optimum performance. Since row bow forms a curvature of the row of magnetic transducers, stripe heights and throat heights can be degraded by its extent. Further, all known methods to contain row bow error or compensate for it are done so at the expense of yield compromise or increased product cost.
It is, therefore, a prime objective in the fabrication of thin film magnetic heads to provide a tooling system and method which reduce row bow to negligible levels. Negligible levels are defined as submicron row bow variability (e.g. 0 microns&lt;row bow tolerance&lt;1.0 microns). By reducing the row bow effect to this extent and employing state of the art electrical lapping technology, magnetic performance is improved by way of improved stripe height and throat height variability. A second manufacturing objective is to achieve the prime objective by providing tooling and processes which are reduced in complexity to minimize cost. A third manufacturing objective is to achieve the prime objective while further reducing cost by maximizing the practical utilization of wafer area to increase productivity. A fourth manufacturing objective is to achieve the prime objective while providing a system for the production of "nano" and "pico" sliders where extreme row bow is anticipated in single row lapping systems.
Methods and systems have been disclosed whereby row bow is corrected by means of a transfer tool with a deflecting beam that corrects for the row bow condition. One example of this type of transfer tool design can be found in U.S. Pat. No. 4,916,868. Another example is shown in U.S. Pat. No. 4,457,114. Although these apparatus can greatly improve the condition of row bow, they fall short of reducing row bow sufficiently to achieve optimum stripe height or throat height variability control.
Alternative methods to correcting beam transfer tools have been proposed in the art, and are summarized in U.S. Pat. No. 5,095,613. This patent describes several prior art approaches that are based on the concept of resisting row bow by geometric strength. This concept is best understood by viewing a row as a mechanical beam. Given a uniform mechanical load by any or all of the row bow mechanisms, the resistance to deflection is proportional to the cube of the height of the row. Therefore, increasing the height of a row, by some means, would decrease the row bow component.
One implementation of this concept is to form rows on wafers in back-to-back pairs and slice them off, as shown in FIG. 1. The rows are separated by a kerf that facilitates subsequent slicing into individual rows and provides increased beam height "H". In order to double lap the row pairs to equalize surface stresses, the row pairs are mounted on a transfer tool, as shown in FIG. 2. One side is lapped, then the pair is debonded and rebonded to lap the other side. When both sides have been lapped, the double row work piece is sliced into two rows along the kerf.
Because slicing the rows in pairs results in an increase in the height of the row beam by over twice the height of a single slider row, it was thought that the double row would resist deflections imposed by bonding, internal wafer stress and the surface stresses initially developed by lapping. However, working with a 100% "large slider" size at 0.85 mm height, this double row method improves the row bow condition but does not achieve the negligible row bow required for optimum slider processing. The sum total of the row bow deflection components exceeds the beam's deflection resistance. With ever smaller sliders being developed, including "nano sliders" (67% slider size) and "pico sliders" (30% slider size), the concept of double row lapping appears to offer little promise.
In another prior art proposal disclosed in U.S. Pat. No. 5,095,613, multiple rows are sliced from a wafer to greatly increase the beam height, as shown in FIG. 3. The multiple rows are bonded to a transfer tool, where the increase in beam height provides for the control of row bow during the bonding process. Each row is lapped and sliced from the work piece until all rows have been lapped. The transfer tool and bond joint limits the row bow condition during the lapping and slicing process. Experiments indicate that nine (9) "large slider" rows, eighteen (18) "nano slider" rows and over forty (40) "pico slider" rows are required before a significant reduction in row bow can be achieved. The major cause of row bow by this method has been found to be shared by wafer stress but dominated by the bond process. The method also requires a departure from previous fabrication methods and requires significant tooling changes.
Applicants herein have determined that the deficiencies of the multiple row method stem from the assumption that Al.sub.2 O.sub.3 -TiC wafer materials would have a low internal stress component and row bow would not exist in a full wafer. It has been found, however, that after slicing the bottom portion from a wafer, as shown in FIG. 3, the wafer stress induced distortion causes a variation in the colinearity of the row of magnetic transducers equivalent to 11.0 um of row bow. Although lower stress wafer mateddais may be available, none have been found to lower stress enough to keep the wafer from distorting or eliminate the row bow effect after removing the bottom portion of the wafer.
Applicants have experimented with alternative systems and mechanisms to mechanically compensate for stress induced wafer distortion. In one system, the top of the wafer is machined into the configuration shown in FIG. 4 by means of electro-static discharge. A mechanical device then applies load forces to distort the top of the wafer in a way that corrects for the row bow distortion. While this system was able to substantially correct for wafer distortion, it occupied the upper area of the wafer where magnetic devices could be placed. Because of this, the proposed system failed to sufficiently reduce cost by maximizing wafer utilization.
It should be evident that many factors must be taken into account in attempting to develop an alternative to single row lapping. It is naive to assume that simply increasing the height of a row beam will eliminate the row bow effect. It is also naive not to consider the manufacturing environment. Therefore, according to the objectives, environment and testing considerations defined above, conclusions can be made regarding prior art solutions.
One conclusion is that the double row lapping system is not a viable approach in the changing environment of decreasing slider size. During tests of double row lapping, it was found that a double row large slider height is not enough height to reduce the row bow to negligible levels. Considering the industry direction toward "nano" and "pico" slider sizes, the double row heights will be decreasing. Since decreasing heights is in opposition to the intent of the method, the double row height concept will not clearly solve the row bow problem and its future is limited.
Another conclusion is that the full wafer concept is not a viable approach because it does not achieve a negligible row bow component. Wafer stress according to studies and tests stated herein caused row bow effects beyond permissible levels. In order for the full wafer concept to be complete and able to achieve the negligible row bow objective, wafer material with stress specifications specific to row bow would have to be provided. Because they have not, practical implementation of this method remains out of reach.
It can be further concluded that the full wafer concept is not a viable approach in achieving the third objective of thin film magnetic head fabrication. The concept requires the upper area of the wafer, as shown in FIGS. 4 and 5, to provide enough material height to resist stress, and enough material area to mechanically hold the work piece until the last row is lapped. In today's environment, population of as much wafer area as possible is essential in productivity to maintain cost competitiveness. The upper area of the wafer can contain a significant amount of product which is clearly a departure from maximum wafer utilization in the third manufacturing objective identified above.
It can be further concluded that the full wafer concept is not a viable approach in achieving the prime objective when using square wafers. As previously discussed, the full wafer concept relies on the upper area material height to maintain constant row bow and mechanically hold the work piece until the last row is lapped. Considering, once again, the third objective of maximum wafer utilization, the last row on a square wafer could be close to or the same height as a single row. If a wafer were populated to this extent, there would not be enough material to maintain constant row bow or mechanically hold the last rows of the work piece during lapping.
Accordingly, there is a need in the art for an alternative to single row, double row and full wafer lapping that can be optimized to achieve all of the competing objectives of thin film magnetic head fabrication depending on the wafer material, tool set and throat height/stripe height tolerance requirements selected. What is required is a solution that accounts for each of the individual components making up the wafer row bow condition and the impact of row bow correction efforts on the overall design objectives.